[zeoblade]

Hi, I'm trying to quantify phosphate in soluble fertiliser by FIA and the reaction is H₃PO₄  +  12H₂MoO₄ --> H₃P(Mo₁₂O₄₀)_(s)  +  12H₂O. 0.9252g of sample was diluted in 0.5L and 0.01L aliquoted and diluted in 0.1L and analysed by spectrophotometre. Standards were prepared so that unknown sample could be interpolated.
According to the absorbance values obtained the triplicate measurements of the phospate in unknown sample have a mean of 9.6ppm when 0.9252g sample is diluted in 0.5L then 0.01L of this is diluted in 0.1L. This is 1 in 500 dilution, right? So the unknown sample is equal to 4777.9ppm of unknown sample.

I'm having a bad mental block. I want to calculate % w/w of phosphorus and P₂O₅. Ppm = ug/mL, correct? If so, I can say 4777.9ug/500mL divided by 0.9252g (using same units) x 100% gives me my % w/w, right? If so, I'm getting 5X10^-1% when I have a target value of 5.5%, I don't feel I'm converting right. I feel I need to consider the MW in the stoichiometric ratio. The absorbance is from H₃P(Mo₁₂O₄₀)_(s) detected which is 1:1 with H₃PO₄

Please critique

[Borek]

phospate in unknown sample have a mean of 9.6ppm

9.6ppm of what? P? H₃PO₄?

ppm is ug/L (that's assuming diluted solution with density close to 1 g/mL, ppm can be ambiguous - see http://www.chembuddy.com/?left=concentration&right=ppm-ppb-ppt).

[zeoblade]

Thanks for replying,

9.6ppm H₃P(Mo₁₂O₄₀)_(s) which is equivalent to H₃PO₄

I'll see if I can get better units than ppm, that's whats causing me the trouble

zeoblade